Conventionally, a spiral separation membrane module is used for a variety of uses such as desalination of brine or sea water, production of ultrapure water, and processing of waste water. As shown in FIG. 7, a spiral separation membrane module has a structure such that a spiral separation membrane element E is housed in a pressure vessel 1, where the gap between the pressure vessel 1 and the element E is shut off with an annular sealing material 4 so as to let the supplied raw liquid 7 flow efficiently into the element E.
The raw liquid that has flown into the element E is subjected to separation with a separation membrane while flowing to the downstream side along a supply-side flow channel material of the element E. The transmitted liquid that has been transmitted through the separation membrane flows along a transmittance-side flow channel material to be collected into a center tube 6. Then, the membrane part of the element E has a structure such that a membrane laminate 2 in which the supply-side flow channel material, the separation membrane, and the transmittance-side flow channel material are laminated is wound in a spiral form around the center tube 6.
Also, at the end part of the element E, there is provided a sealing material holding member 10 that holds the annular sealing material 4 on the outer circumferential part thereof. The sealing material 4 is made of rubber or the like having a cross-section with a U-letter or V-letter shape, and widens to the outside by the pressure at the time of receiving a water flow, so as to seal the gap between the membrane element E and the pressure vessel 1 in that state (see, for example, Patent Document 1)
At the center of the sealing material holding member 10, there is provided a central opening part 19 into which the center tube 6 is inserted. The inner circumferential surface thereof has a cylindrical form having a flat surface or a cylindrical form having a plurality of convex streaks (ribs) in the axial direction. Then, the height of the convex streaks and the like are designed so as to have a dimension such that the outer circumferential surface of the center tube 6 inscribes the inner circumferential surface or the convex streaks of the central opening part 19.
However, there are cases in which the outer diameter dimension of the center tube 6 varies during the production steps. In particular, it has been made clear that, between Japan and the United States, even if the outer diameter has an identical designed dimension, there may occur a difference of as large as about 0.8 mm in terms of an average value of the outer diameter. When clearance is increased in consideration of such a variation, the center tube 6 is liable to become eccentric relative to the sealing material holding member 10, thereby aggravating the outer appearance and degrading the commercial quality of the product in some cases. Also, when the center tube 6 is made eccentric, the operation of attaching the membrane element may become cumbersome depending on the structure of the pressure vessel 1, thereby raising a problem in that the connection operation may become cumbersome in connecting a plurality of membrane elements for use, or the like problem.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 11-267470.